ALBERT

Description: S-net is a large-scale seafloor network in the Japan Trench area, consisting of inline-type 150 observatories with seismometers and pressure gauges. We analyzed the strong-motion data recorded at the S-net sites from earthquakes with magnitudes between about 4 and 7 with focal depths lower than 70 km. To minimize the effects of nonlinear site response and weak-couplings between the seabed and sensor houses, recordings with vector peak accelerations lower than 50 gals were selected. More than 6,000 recordings with high S/N ratios from about 600 earthquakes were used. Using the spectral inversion technique, we obtained some fundamental properties of the earthquake source spectra, path attenuation, and site factors from the horizontal-component S-wave portions of the recordings. The source spectra followed the w〈sup〉-2〈/sup〉 source model generally well, and the estimated magnitudes were similar to the catalog magnitudes. The stress drops increased systematically with the focal depths, and the stress-drop values differed between the earthquake types. The path-averaged quality factors were somewhat larger than those in the past studies. The peak site frequencies ranged between about 0.2 and 10 Hz, while the peak amplification factors were between about 10 and 50. A moderate regionality was also observed in the distribution of site amplification factors. At some stations which were not buried, spurious site spectra were recognized. We discuss the above results and their implication in the presentation.